My favorite biological saying is this: “the most common surface for an animal to live on is another animal.” This description obviously defines parasites. Humans, for example, are home to at least 300 parasites. It turns out, parasitizing another species is an extremely successful evolutionary strategy.
Dog owners are constantly reminded by veterinarians about the dangers of heartworm, which can be fatal. These parasites are transmitted by mosquitoes, which are considered “intermediate hosts.” An uninfected female mosquito bites an infected dog and ingests microscopic baby worms, which mature into infective larvae after about 2 weeks. The mosquito bites a new dog and the heartworm larvae enter the bite wound and find their way to the right side of the heart where they mature into adults. Adult female worms can reach 10-12 inches (males 5-6) and produce new microscopic baby worms that travel in the blood stream hoping to be sucked up by a mosquito. Just thinking about my dogs having a 12-inch worm in their hearts keeps me motivated to give them a monthly dose of heartworm-preventative.
While many parasites can impact a host’s health, there’s another class of parasites that can actually affect the behavior of their hosts. They are in fact known as “mind-control” parasites, and their study is part of a developing field known as “neuro-parasitology.”
A well-known behavior altering parasite is Toxoplasma gondii. It is a protozoan parasite that infects and reproduces inside cells of its hosts, mostly warm-blooded animals. The definitive hosts are members of the Felidae (domestic cats and relatives). Here they reproduce in the intestines and produce “oocysts” that are shed in feces. To the parasites’ advantage, cat feces contain millions of oocysts.
Although T. gondii and its negative effects have been known to science for over 100 years, only recently have some of its more insidious effects become apparent, in particular, how it affects wolf behavior.
Wolves are infected by T. gondii the same way other animals are infected. Sometimes infections occur when wolves kill and eat mountain lions, especially kittens, or scat (now you know where your dog got that bad habit). A scientific study by Connor Meyer and Kira Cassidy, based on wolves in Yellowstone Park over 26 years recorded wolf behavior and the presence of T. gondii. Male wolves infected with T. gondii exhibit heightened risk-taking behavior. Meyer and Cassidy found that infected wolves are 11 times more likely to leave their natal pack and move to new territory, which in itself is a risky behavior. These wolves are 46 times more likely than an average male to become pack leaders. T. gondii apparently gives them an attitude that other males choose to avoid. What’s in it for the parasite is less clear.
It doesn’t seem that making a male wolf more aggressive is to the parasites’ advantage because wolves are unlikely to pass the parasite back to mountain lions or other wild felines. However, a quick online search (e.g., totally unscientific) suggests that foxes and coyotes eat wolf scat, so perhaps there is some advantage to the parasite. Alternatively, risk-averse behavior might make wolves more likely to get hit by cars, a lose-lose for wolf and parasite.
T. gondii does not just alter wolf behavior. Infected mice lose their fear of cats, with obvious benefit to the parasite. Toxoplasma-infected captive chimps lose their aversion to leopard urine, also a lose-win for those two. It is likely that in the future we will find an even broader array of animals, including us, that have their behavior altered by T. gondii.
If you needed another reason to advocate for keeping house cats inside, here’s another. House cats on the loose are known spreaders of T. gondii. And fyi, it’s a felony to shoot a cat in Minnesota, even one that’s feral.